The findings, which are published in the journal Science by University of Alaska Fairbanks researcher Natalia Shakova and her colleagues, suggest that the East Siberian Arctic Shelf, which covers an area of about 2 million square kilometres, is releasing more methane than the rest of the world's oceans put together.

But unlike other methane sources, where the gas is produced by bacteria breaking down organic matter in sediments, the Siberian methane deposits are pre-made and sequestered under the sea as so called hydrates - gas-filled watery cages.

This means that any event that destabilises the deposits can cause them to release their stored methane.

The team made the discovery by mounting a series of expeditions to collect water and air samples from the Siberian coastline. What they found were very high concentrations of the gas in the seawater, arguing for an undersea source.

The concern is that, with global warming, more fresh water entering the area from melting permafrost will cause local sea temperatures to rise, further destabilising the methane deposits and causing the seafloor to surrender its stored gas in a series of convulsive oceanic belches.

This would dramatically accelerate the rate of global warming because methane is at least 30 fold more potent as a greenhouse gas than the equivalent amount of CO2.

"The release to the atmosphere of only one percent of the methane assumed to be stored might alter the current atmospheric burden of methane up to 3 to 4 times," says Shakova. "The climate consequences would be hard to predict..."

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